Download A STUDY OF ELECTROGRAMS RECORDED FROM THE CONUS

A STUDY OF ELECTROGRAMS RECORDED FROM THE CONUS
ARTERIOSUS OF AN ELASMOBRANCH HEART*
By A. K. TEBECIst
Electrograms of fish have been briefly described by Kisch (1948, 1949) and
Oets (1950). Both authors reported an additional excursion (B wave) on the ST
segment, corresponding to contraction of the conus arteriosus. This communication
gives a more detailed account of this event and of its relationship to the behaviour
of the conus.
Port Jackson sharks (Heterodontus portusjacksoni) were anaesthetized by
intravenous injection of Tricaine (MS222, Sandoz) (4-6 mg/lb body wt.) and mounted
ventral side uppermost in a stainless steel tank containing constantly circulating
sea-water. The air-conditioned room was maintained at a temperature of 17 ±1 °C.
The heart was exposed within the pericardium. Conal electrograms were recorded
by means of a unipolar silver electrode to which was attached a 1·5-cm length of
cotton thread moistened with elasmobranch Ringer, and were displayed on an
oscilloscope.
Results
Conal electrograms (positivity upwards) are illustrated in Figures 1 and 2.
Wave complexes were identified by comparing them with those of ventricular
electrograms recorded simultaneously. These wave forms varied considerably from
animal to animal, in some preparations being entirely monophasic (either positive
or negative) but generally being polyphasic to various extents. However, in all
electrograms, the P, QRS, and T complexes were readily identified. In addition,
conal electrograms exhibited a large excursion in the ST segment, corresponding
to conal contraction. This is designated as the conal depolarization complex (Ba).
In some preparations (Fig. 1) a prominent wave occurred between the T wave of
one cardiac cycle and the P wave of the next. This corresponded to conal diastole
and accordingly, is designated as the conal repolarization complex (B r ). Why a
Br wave was not always present is not clear. Generally, the Ba complex occurred
before the T complex, but in three preparations the two wave forms occurred
simultaneously, or at least, overlapped. Figure 2 is of such a preparation. Here
the Ba and T complexes overlapped in electrograms recorded ~ 3·5 mm from the
conus-ventricular (CV) junction (Fig. 2).
* Manuscript received March 17,1967.
t Department of Zoology, University of Adelaide; present address: Physiology Department, Monash University, Clayton, Vic.
Aust. J. bioI. Sci., 1967,20, 843-6
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The electrograms presented in Figure 2 were recorded at a series of distances
distal to the CV junction. It is clear from the figure that the conal depolarization
complex occurs later in the ST segment with increasing distance from the CV
junction; its onset is marked by a dot. The onset of the "intrinsic deflection"
probably represents the wave of excitation passing beneath the recording electrode
(Hoffman and Cranefield 1960). A plot of the times of arrival of excitation with
position on the conus did not reveal a specific pattern of excitation spread. Excitation
appeared to emanate at the CV junction and travel diffusely to the rostral end.
p
QRS
Bd
T
Br
>e
<:'I
o
0-5 sec
Fig. I.-Conal electrogram in which the conal repolarization (Brl wave was prominent.
By calculating the time delay between the intrinsic deflections of two conal
electrograms recorded at a measured distance apart, an approximate conduction
velocity of 2-4 cm/sec was determined. Similarly (using the intrinsic deflection
of the QRS complex), the conduction velocity of surface excitation in the ventricle
was determined (40-100 cm/sec). For a more accurate value, it is necessary to
determine the fibre length between the two points of recording. However, the
approximate values given above serve to indicate the relative magnitudes of
conduction velocity between the ventricle and conus.
The PR value represents the time for excitation to travel from the atrium
to the point of recording on the ventricle. PR values of ventricular electrograms
recorded on the immediate proximal side of the CV junction were 50-160 msec
shorter than those of conal electrograms immediately distal to the CV junction.
This indicates that there is a delay in excitation spread across the CV junction.
Discussion
The conus arteriosus of the Port Jackson shark is a relatively thick-walled,
muscular, barrel-shaped chamber containing three tiers of semilunar valves. The
results of this investigation suggest that the electromechanical properties of conal
tissue enable the conus to function according to the hypothesis proposed by Satchell
and Jones (1967).
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Distance from
CV Junction
(mm)
o
2'0
p
Q RS
T
p
'~
.....
2'5
3·5
5·5
6'0
7'0
---..;~
8'5
10'0
11'0
'-~
-~
12'5
>
E
....
o
1 sec
Fig. 2.-Conal electrograms recorded at various distances from the conusventricular (CV) junction. The conus in this preparation was 13 mm long.
The dot indicates the beginning of the slow "intrinsic deflection" of the Bet
complex.
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846
Excitation spreads relatively quickly over the ventricle, which contracts
synchronously. There is a delay at the CV junction, after which excitation travels
diffusely forward over the conus. This delay prevents conal systole from occurring
too soon after ventricular systole and so enables the ventricle to expel its contents
with a minimum of resistance. The spread of excitation in the conus is extremely
slow (compare values for heart muscle by Schaefer and Trautwein 1949; Brendel,
Raule, and Trautwein 1950; Curtis and Travis 1951), causing the conus to contract
slowly in a peristaltic wave which begins at the CV junction and moves forward.
Since the Bd wave occurs late in the ST segment, the lower conus must contract
during late ventricular systole. Similarly, the short time delay between the Br and
T waves in the upper conus indicates that this portion must contract just before,
or as, the ventricle relaxes. By means of blood pressure and flow recordings, Satchell
and Jones (1967) showed that the lower conal valves close first, before the end of
ventricular systole, and that the upper conal valves close at the end of conal systole.
They proposed the hypothesis that the conus serves to postpone the closure of the
upper valves until the negative intrapericardial pressure, caused by ventricular
ejection, has decreased to a lower value. The sequence of events indicated by conal
electrograms is in accord with this hypothesis. The low conduction velocity of conal
muscle accounts for this.
I am very grateful to Professor G. H. Satchell for his valuable comments.
References
BRENDEL, W., RAULE, W., and TRAUTWEIN, W. (1950).-Pflugers Arch. ges. Physiol. 253, 106.
CURTIS, H. J., and TRAVIS, D. M. (1961).-Am. J. PhY8iol. 165, 173.
HOFFMAN, B. F., and CRANEFIELD, P. F. (1960).-"Electrophysiology of the Heart." (McGraw.
Hill Book Co., Inc.: New York.)
KISCH, B. (1948).-Expl. Moo. Surg. 6, 31.
KISCH, B. (1949).-Expl. Med. Surg. 7, 55.
OETS, J. (1950).-PhyBiologia compo Oecol. 2, 181.
SATCHELL, G. H., and JONES, M. P. (1967).---J. expo Biol. 46, 373.
SCHAEFER, H., and TRAUTWEIN, W. (1949).-Pflugers Arch. ges. PhY8iol. 251, 417.
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